Identification marking of products is of increasing importance in almost every branch of industry. By way of example, production data, expiry dates, barcodes, company logos, serial numbers, etc. often have to be applied to said products. These markings are often currently still produced by conventional techniques, such as printing, embossing, stamping, and labeling. However, increasing importance is being placed on contactless, very rapid and flexible marking by lasers, in particular for plastics products and plastics packaging. This technique permits high-speed application of identification marks, e.g. graphic inscriptions, such as barcodes, even when the surface is not flat. The location of the inscription is within the actual plastic and it is therefore durable, abrasion-resistant, and counterfeit-resistant.
Alongside CO2 lasers, Nd-YAG lasers and excimer lasers are increasingly used for laser identification marking of plastics. However, laser marking of many plastics is difficult or impossible unless they are subjected to additional modification, examples being polyolefins and polystyrenes. By way of example, a CO2 laser emitting light in the infrared region at 10.6 μm produces only a weak, barely legible marking on polyolefins and polystyrenes, even if very high power levels are used. Again, Nd-YAG lasers do not interact with polyurethanes and polyetheresters. In order to achieve a good marking, complete reflection or complete transmission of the laser light by the plastic is not permissible, because no interaction then occurs. In contrast, if energy absorption is excessive, the plastic can vaporize in the irradiated region, the result then being an engraving process rather than a visible graphic identification marking.
It is known that plastics can be rendered laser-inscribable by adding appropriate absorbers, e.g. absorbent pigment particles. The incident laser light is then converted to heat, producing a marked rise in the temperature of the pigment particles within fractions of a second. If the amount of heat generated by absorbed laser light within a very localized region of the plastic is greater than can be simultaneously dissipated into the environment, the result is severe local heating and either carbonization of the plastic or a chemical reaction of the plastic and/or of the absorber, or formation of a gas, e.g. CO2. These processes lead to alteration of color, transparency, and/or refractive index for incident light, and thus to marking of the plastic.
It is known that organic dyes and pigments can be used for modification of plastics in order to render these laser-markable.
EP-A-684144 A1 describes a composition for the coloring of plastic parts via laser irradiation, comprising a mixture of at least one opacifier and at least one chromogenic compound. The opacifiers here were selected from mica, nanoscale TiO2, and metal oxides based on antimony oxide.
WO 01/00719 A1 describes a polymer composition which comprises a polymer and an additive that permits laser marking. Antimony trioxide is used here at a concentration of at least 0.1% by weight, with particle size>0.5 μm.
WO 95/30546 A1 describes laser-markable plastics, in particular thermoplastic polyurethanes, which comprise pigments, where these were coated with doped tin dioxide.
WO 2006/065611 A1 describes a material made of a thermoplastic polyurethane, of an opacifying bismuth oxide, and, if appropriate, of a colorant, and the use of said material for laser marking.
The abovementioned processes have at least one of the following disadvantages:                The pigments used comprise those with particle sizes in the μm range (≧1000 nm), which therefore exhibit considerable scattering in the visible region of the spectrum, or those which color the plastics matrix, or have an intrinsic color, or are used at concentrations that produce opaque coloration. This makes it impossible to inscribe transparent plastics.        The absorption of the dopant is non-maximal at the laser wavelength used for the identification marking process. A relatively high amount of the dopant is therefore required, often above 0.1%.        The use of antimony-containing dopants is undesirable, because of the undesirable contamination of the doped plastics with heavy metal.        Some of the pigments used hitherto as dopant are electrically conductive, an example being antimony-doped tin oxide. Use of these increases the conductivity of the plastic and thus lowers its tracking resistance, and this is disadvantageous for particular applications.        